Abstract
Ionic zinc (Zn) has been reported to enhance binding of human growth hormone (hGH), but not prolactin (PRL), to the human PRL receptor at a concentration of ~50uM (1). We have investigated the effect of zinc on hGH bioactivity using a lactogenic bioassay. The potencies of selected doses of both hGH and PRL in the presence of increasing concentrations of ZnCl2 were investigated with an Eluted Stain Assay (ESTA) which used Nb2 cells. This colorimetric bioassay is based upon the reduction of a yellow tetrazolium salt, MTT, to its purple formazan by lactogen-activated Nb2 cells. Zinc enhanced the bioactivity of hGH but not PRL. Progressive enhancement of a low dose (0.5mU/L; 9pM) of hGH (IRP 80/505) was observed over the Zn concentration range of 6-100uM; higher Zn concentrations were inhibitory. Potentiation of bioactivity was observed only with low doses of hGH; with >2.5mU hGH/L, 50uM Zn inhibited the response. The bioactivity of PRL was consistently inhibited by 50uM Zn (PRL range 0.2-50mU/L; 0.4-105pM), inhibition being greater with higher PRL doses. We conclude that in this precise and sensitive bioassay, zinc has a differential effect on hGH and PRL bioactivities which concords with the radioligand studies but potentiation of bioactivity was only observed with low hGH doses. It was optimal at ~50uM Zn but was far less than might have been anticipated from the binding studies, when the affinity increased by 8000-fold. This discrepancy was not explained by significant endogenous Zn in the bioassay medium (only 2uM by Atomic Absorption Spectroscopy). Our findings therefore do not support the earlier suggestion (1) that Zn is crucial for the lactogenic bioactivity or hGH. (1) Cunningham B.C, Bass S, Fuh G, Wells JA 1990 Science 250:1709-1712.
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